Since their introduction in the early 1980s, microchannel heat sinks have shown much potential for high heat-flux cooling applications and have been used in the industry. However, existing microchannels include conventional parallel channel arrangements which are not well suited for cooling heat producing devices which have spatially-varying heat loads. Such heat producing devices have areas which produce more heat than others. These hotter areas are hereby designated as “hot spots” whereas the areas of the heat source which do not produce as much heat are hereby termed, “warm spots”.
FIGS. 1A and 1B illustrate a side view and top view of a prior art heat exchanger 10 which is coupled to an electronic device 99, such as a microprocessor via a thermal interface material 98. As shown in FIGS. 1A and 1B, fluid generally flows from a single inlet port 12 and flows along the bottom surface 11 in between the parallel microchannels 14, as shown by the arrows, and exits through the outlet port 16. Although the heat exchanger 10 cools the electronic device 99, the fluid flows from the inlet port 12 to the outlet port 16 in a uniform manner. In other words, the fluid flows substantially uniformly along the entire bottom surface 11 of the heat exchanger 10 and does not supply more fluid to areas in the bottom surface 11 which correspond with hot spots in the device 99. In addition, the temperature of liquid flowing from the inlet generally increases as it flows along the bottom surface 11 of the heat exchanger. Therefore, regions of the heat source 99 which are downstream or near the outlet port 16 are not supplied with cool fluid, but actually warmer fluid or two-phase fluid which has already been heated upstream. In effect, the heated fluid actually propagates the heat across the entire bottom surface 11 of the heat exchanger and region of the heat source 99, whereby fluid near the outlet port 16 is so hot that it becomes ineffective in cooling the heat source 99. This increase in heat causes two-phase flow instabilities in which the boiling of fluid along the bottom surface 11 forces fluid away from the areas where the most heat is generated. In addition, the heat exchanger 10 having only one inlet 12 and one outlet 16 forces fluid to travel along the long parallel microchannels 14 in the bottom surface 11 for the entire length of the heat exchanger 10, thereby creating a large pressure drop due to the length the fluid must travel. The large pressure drop formed in the heat exchanger 10 makes pumping fluid to the heat exchanger 10 difficult and augments the instabilities.
FIG. 1C illustrates a side view diagram of a prior art multi-level heat exchanger 20. Fluid enters the multi-level heat exchanger 20 through the port 22 and travels downward through multiple jets 28 in the middle layer 26 to the bottom surface 27 and out port 24. In addition, the fluid traveling along the jets 28 does not uniformly flow down to the bottom surface 27. In addition, the heat exchanger in FIG. 1C exhibits the same problems discussed above with regard to the heat exchanger 10 in FIGS. 1A and 1B.
What is needed is a heat exchanger which is configured to achieve a small pressure drop between the inlet and outlet fluid ports while efficiently cooling the heat source. What is needed is a microchannel heat exchanger which is configured to achieve proper temperature uniformity in the heat source. What is also needed is a heat exchanger which is configured to achieve proper temperature uniformity in light of hot spots in the heat source.